We are extending the structural investigations of McPC 603 to other immunoglobulin Fabs. Crystals have been grown for EPC 109 and TEPC 601, two mouse myeloma proteins with binding specificities for levans and galactans, respectively. Some heavy atom derivatives have been obtained for J539, another galactan specific protein, and their positions in the crystal determined. The three-dimensional structure of McPC 603 has been used as a basis for understanding the sequence variations in other proteins with similar binding specificities. The most probable interpretation of these data is that there is only one binding site for phosphorylcholine in these proteins. A model of BS5, an anti-type III polysaccharide antibody from the rabbit, has been constructed. The model has a large cleft in which it is possible to bind a hexasaccharide molecule.